The present invention relates to a scanning electron microscope for observing a miniature pattern to measure dimensions thereof, and more particularly, to a scanning electron microscope for observing and measuring a sample, the shape of which can be deformed by an electron beam irradiated thereto.
Scanning electron microscopes (SEM) are widely used in manufacturing and testing steps of a functional product such as a semiconductor device, a thin film magnetic head, and the like, which are fabricated by micro-machining the surface thereof, for measuring widths of processed patterns and inspecting the appearance of resulting products. The scanning electron microscope is an apparatus for forming the image of a sample by narrowing down an electron beam emitted from an electron source with a converging lens or an objective lens which makes use of an interaction of a magnetic field or an electric field with the electron beam, one-dimensionally or two-dimensionally scanning the electron beam on the sample using a deflector, detecting a secondary signal (secondary electrons, reflected electrons, or electromagnetic waves) with a detector which makes use of an opto-electric effect or the like, and converting the detected signal into a viewable signal such as a luminance signal synchronized to the scanning of the electron beam. Considerable efforts have been put into the scanning electron microscope to provide a sample image which accurately corresponds to the shape of the surface of the sample under observation and length measurement, and the distance between arbitrary two points is calculated on the surface of the sample from the sample image thus generated. This calculation is commonly called “length measurement,” and a scanning electron microscope having such a calculation function is called a “length measuring SEM.” Such a scanning electron microscope irradiates the surface of a sample under observation with an electron beam having accessible energy of several hundreds of electronvolts, as a matter of course.
On the other hand, further miniaturization has been advanced in recent years in the micro-machining on the surface of semiconductor, and a photoresist which reacts to argon fluoride (ArF) excimer laser light (hereinafter called the “ArF resist”) has been used for a photosensitive material of photolithography. Because of its wavelength as short as 193 nm, the ArF laser light is regarded as suitable for exposure to more miniature circuit patterns. However, the results of recent investigations have revealed that the ArF resist is highly vulnerable to electron beam irradiation, and when a formed pattern is observed or measured with a scanning electron microscope, the scanning of a converged electron beam causes a condensation reaction in a base acrylic resin or the like, resulting in a reduction in volume (hereinafter called “slimming”) and an eventual change in the shape of a circuit pattern.
It is said that for reducing the slimming of the ArF resist, it is effective to reduce an irradiation density of an electron beam to a sample. However, a reduction in the irradiation density of an electron beam causes a reduction in the amount of secondary electrons generated from the sample, resulting in a dark image. Therefore, there is a need for a method of limiting the slimming and generating a highly visible image.
For generating a highly visible image, an optimal brightness and contrast must be defined for a sample image. In the prior art, the brightness and contrast of a sample image are adjusted by changing condition settings for a detector and an amplifier. JP-A-7-240166 describes a contrast adjusting method using a contrast level conversion function.
In the prior art method mentioned above, when a micro-machined ArF resist pattern is measured twice under the conditions of a frame count equal to 16, and a probe current amount equal to 24 pA, a slimming amount of 1.3 nm occurs between the first and second length measurements, demonstrating a failure in sufficiently reducing the slimming in the length measurement of the micro-machined ArF resist pattern. Disadvantageously, the prior art method of reducing the slimming does not take into consideration an image control technique which relies on the relationship between the number of times of electron beam scanning required for creating a sample image (hereinafter called the “frame count”) and the probe current amount, and fails to sufficiently reduce the slimming and generate a highly visible sample image.